Roaming pertains to using mobile services
anywhere the subscriber goes. For roaming to other countries, the subscriber needs only
subscribe to the international roaming service. He uses the same Mobile Equipment (ME) to
access the services of different networks on a regional, national, and/or an international
level.
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Subscriber Identification Module (SIM)
roaming occurs in cases where the handset is incompatible due to difference in radio
technology. The subscriber takes his SIM card containing his personal data and inserts it
in a rented ME, which is able to transmit and receive on the network he or she is roaming
in. An example is the usage of different frequency bandwidth like GSM 900, DCS/PCN also
known as GSM1800, PCS 1900 in the US. Of course, it is also possible to get a dual-band MS
for DECT-GSM, AMPS-GSM, and GSM-PCN. Later on, multi-mode would be feasible.
Simply equipped with a single number and a
SIM card, the subscriber unlocks the world of telecommunication at his disposal. The
object of identification is focused on the SIM card.
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GSM MoU
The GSM Memorandum of Understanding (MoU)
Association represents the interests of administrative, regulatory network operators. Its
main responsibility is to forge a commercial agreement between potential operators who
would commit themselves to implementing standards in a given time frame to upkeep global
GSM services.
One of the working groups, the
International Roaming Expert Group (IREG) is primarily responsible for international
roaming issues. These documents describe the following aspects:
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-
Network operator
databases -
SCCP signaling aspects,
optimized routing -
End-to-end functional
capability test -
Included are also services
aspects when roaming.
The specification is for Phase I and Phase
II basic services, supplementary services, SMS, data, and fax.
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Testing Needs
In order to guarantee correct
international roaming operation, one solution would be to implement test cases as
stipulated by IREG.
The needs of testing arise from the
following situations:
-
Network configuration changes
due to introduction of new services and inclusion of new roaming partners -
Modifications in addressing and
numbering data for routing -
Addition of new network
elements such as MSC, VLR, HLR, EIR, etc. -
Major SW upgrade of SS#7
signalling on the protocols like SCCP, MAP, and ISUP.
Any changes on either side, the home
Public Land Mobile Network (PLMN) and/or its roaming partner mean renewed testing.
Furthermore, periodic verification for detection of problems and potential alarm errors
are necessary preventive measures. The most obvious testing method is manual. This means
to have real mobile phones with international SIMs and fixed telephone lines and then
literally making calls in all possible test configurations. The current testing needs
calls for more automated testing solutions.
The better solution is to stimulate some
parts of the network, i.e., replacing the handphones and telephones with a system
generating such calls.
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The air interface is excluded from the
test environment. Therefore, the BSS part of the network is simulated. This prevents air
interface errors and allows re-runs of tests to reproduce errors detected.
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alt="Viraj Pradhan, is business manager (telecom), Tektronix India Ltd" align="right"
hspace="6" vspace="6">Since BSC and MSC is based on the SS#7
network, the underlying SS#7 layers of SCCP must also be tested. MAP is tested, as it is
the protocol used in MSC procedures. Included are also supplementary services like call
forwarding and call barring.
A list of roaming PLMNs is available. At
least three subscriber profiles of a PLMN roaming partner are stored in a configuration
file.
The execution of the test cases is
automatic with an unlimited number of roaming partners. Selective test cases and roaming
partners are possible. The starting of the tests can be pre-scheduled at low traffic time,
e.g., at night. The system stores the data of each roaming partner. During live testing,
it can be left unattended. The signalling data together with the test results are stored
on disk for later review.
The testing time for each roaming partner
is greatly reduced. Thus, the network operator can afford to run the test cases repeatedly
any time.